The invention relates to an arrangement for synthesizing speech from a band of low-frequency components of a speech signal and a plurality of narrowband control signals which are characteristic of a plurality of sub-bands of high-frequency components of the speech signal, comprising means for generating a band of high-frequency components from the band of low-frequency components, means for dividing the band of high-frequency components into a number of sub-bands corresponding to the sub-bands of high-frequency components of the speech signal, means for correcting, by means of the control signals, the sub-bands derived from the generated band and means for combining the band of low-frequency components with the corrected sub-bands of the generated high-frequency components to a speech output signal.
Arrangements of such a type are used as speech-synthesizing arrangements in voice-excited vocoders. Voice-excited vocoders can be distinguished into channel vocoders and formant vocoders, depending on the manner in which the sub-bands of high-frequency components are chosen and on the character of the control sgnals derived therefrom. For channel vocoders the starting point is a, usually rather large, number of contiguous sub-bands from which control signals are derived which are a measure of the average signal amplitude in each sub-band. The arrangement described in U.S. Pat. No. 3,139,487 may be considered an example of such a channel vocoder. For formant vocoders the sub-bands are formed by a small number, usually three or four, formant ranges, the control signals supplying information about the frequency and the amplitude of the spectral peaks occurring in a formant range. An example of such a formant vocoder is described in J. L. Flanagan, "Resonance-vocoder and baseband complement", IRE Transactions on Audio AU-8, 1960, pages 95-102.
Such vocoders utilize a distortion network for the generation of a band of high-frequency components from the band of low-frequency components. Known simple distortion networks such as limiters and rectifier circuits were not very satisfactory since they resulted in speech output signals which sound unnatural or at least less natural. Consequently very complicated distortion networks have been designed. In this connection reference is made to, for example, M. R. Schroeder and E. E. David Jr., "A vocoder for transmitting 10 kc/s speech over a 3.5 kc/s channel", Acustica No. 10, 1960, pages 35-43, FIG. 5 in particular.